The invention relates to a piezoelectric actuator, in particular for actuating control valves or injection valves in motor vehicles, having an actuator body in the form of a multilayered laminate made up of stacked layers of piezoelectric material with intervening metallic or electrically conductive layers that function as electrodes, and the end faces of this actuator body are respectively contacted by a top plate oriented toward the valve having an axially protruding valve tappet and a bottom plate at the opposite end, wherein the actuator rests in an axial bore of a valve housing and is prestressed at the ends in the axial direction between the top plate and the bottom plate by means of a spring element.
A piezoelectric actuator of this kind has been disclosed, for example, by DE 196 50 900 A1 from Robert Bosch GmbH.
If a pulsating electrical voltage is applied to their electrode layers, multilayered piezoelectric actuators of this kind execute analogously pulsating strokes by changing the distance between their two end faces. The multilayered piezoelectric actuator is operated with voltages that produce high field intensities. Field intensities of 2 kV/mm to 3 kV/mm are customary. The operating voltage lies between 100 V and 250 V. The multilayered piezoelectric actuator must be supported in the steel housing of the injection valve in such a way that no short-circuits occur. A centered installation with a definite distance to the outer wall is consequently required so that there is no danger of short-circuits between the actuator body and the outer wall.
During operation of multilayered piezoelectric actuators of this kind, heat is produced in the actuator body which must be dissipated outward, i.e. to the valve housing of the injection or control valve, for example, in order to prevent damage to the piezoelectric actuator and impairment of its function.
In the above-mentioned DE 196 50 900 A1, in order to center the actuator inside the valve housing, it is clamped between its top plate and its bottom plate by means of a spring sleeve, which is under a great deal of tensile stress, in such a way that the top plate and the bottom plate subject the actuator body to a corresponding initial compressive stress. In this instance, the spring sleeve absorbs the heat generated by the actuator body, dissipates it outward, and simultaneously provides for the centering of the actuator body in the valve housing required for the insulation.
According to the patent DE 197 15 488 C1 from Siemens AG, an actuator body is enclosed by a polymer casing which has an open air gap between itself and the inner wall of the valve housing. This produces the disadvantage that the heat dissipation to the valve housing is impeded by the air gap.
Object and Advantages of the Invention
In view of the above mentioned, the object of the invention is to produce a piezoelectric actuator of this generic type, which is protected against contamination and moisture even before installation in the valve housing, in which a simple centering of the actuator in the valve housing is achieved. Simultaneously, an electric insulation of the actuator to the valve housing and favorable heat transmission from the actuator to the valve housing are achieved.
According to an essential aspect of the invention, the actuatorxe2x80x94at least including the top platexe2x80x94is enclosed by an electrically insulating elastomer sleeve so that the actuator with the elastomer sleeve rests snugly against the inner wall of the axial bore of the valve housing in the vicinity of the actuator. Before insertion of the actuator into the axial bore, this elastomer sleeve preferably has a convex shape whose diameter exceeds the inner diameter of the axial bore of the housing over the entire axial length of the actuator body. As a result, the actuator is centered according to the invention over the entire length of the actuator body after insertion into the valve housing, is electrically insulated from the valve housing, and is protected against contamination and damage before installation. At the same time favorable heat transmission from the actuator body to the valve housing is assured. In the piezoelectric actuator according to the invention, at least the bottom plate is integrated with the actuator body by means of the elastomer sleeve and consequently, the elastomer sleeve adhering to the actuator body and to at least the bottom plate leads to a better centering during installation. The integration of the top and bottom plate also prevents a rotation of the actuator so long as at least the bottom plate is secured in the valve housing.
If the top and bottom plate are completely or partially made of electrically insulating material, then the short-circuit danger on the top and bottom of the actuator can be considerably reduced. Even with a metallic top and bottom plate, the encapsulation within the elastomer sleeve greatly improves the short-circuit safety.
Intentional air inclusions in the elastomer can, if necessary, be used to reduce compressive strains produced by thermal expansion.
If necessary, the centering of the actuator body can be improved by means of inserted centering rings made of a stronger polymer.
Since the elastomer sleeve also encompasses the mobile top plate, it is used there to center and guide the actuator without producing abrasion between the top plate and the valve housing.
The actuator which is encapsulated by the elastomer sleeve before installation can be more favorably and reliably transported and installed.
Optionally, the elastomer sleeve is filled with thermally conductive supplementary particles which increase the thermal conductivity of the elastomer sleeve and thereby improve the dissipation of the heat produced during operation of the actuator body.
Other ductile polymers can also be used in lieu of the elastomer sleeve.
The above-mentioned advantages and other advantageous features will be clarified in the following description of several exemplary embodiments of a piezoelectric actuator according to the invention when this description is read in conjunction with the drawings.